In an effort to understand the mechanisms by which nonsense codons affect RNA metabolism in mammalian cells, nonsense mutations were generated within the gene for the secretory major urinary protein (MUP) of mice. The translation of MUP mRNA normally begins within exon 1 and terminates within exon 6, the penultimate exon. Through the use of Northern (RNA) blot hybridization and assays that couple reverse transcription and PCR, a nonsense mutation within codon 50 of exon 2 or codon 143 of exon 5 was found to reduce the abundance of fully spliced, nuclear MUP mRNA to 10 to 20% of normal without an additional reduction in the abundance of cytoplasmic mRNA. In contrast, a nonsense mutation within codon 172 of exon 5 was found to have no effects on the abundance of MUP mRNA. These findings suggest that a boundary between nonsense mutations that do and do not reduce the abundance of nuclear mRNA exists within the exon preceding the exon that harbors the normal site of translation termination. In this way, the boundary is analogous to the boundary that exists within the penultimate exon of the human gene for the cytosolic enzyme triosephosphate isomerase. Assays for exon skipping, i.e., the removal of an exon as a part of the flanking introns during the process of splicing, reveal that 0.1, 2.0, and 0.1% of MUP mRNA normally lack exon 5, exon 6, and exons 5 plus 6, respectively. Relative to normal, the two nonsense mutations within exon 5 increase the abundance of RNA lacking exon 5 on average 20-fold and increase the abundance of RNA lacking exons 5 plus 6 on average 5-fold. Since only one of these nonsense mutations also reduces the abundance of fully spliced nuclear mRNA to 10 to 20% of normal, the two mechanisms by which a nonsense mutation can alter nuclear RNA metabolism must be distinct. The analysis of missense mutations within codons 143 and 172, some of which retain the nonsense mutation, indicates that the reduction in the abundance of fully spliced nuclear mRNA is dependent upon the premature termination of MUP mRNA translation, whereas skipping is attributable to nonsense mutation-mediated changes in exon 5 structure rather than to the premature termination of translation. The increase in exon 5 skipping by either the nonsense or missense mutations within codon 172 correlates with a decrease in the complementarity of exon 5 to U1 snRNA. This suggests that a 5' splice site may extend as far as 12 nucleotides into the upstream exon, which is, to our knowledge, the largest extension.